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9OO                              Appendix II.

Constructive defects occur less often nowadays, the necessity
for both strength and elasticity being fully recognised, good mild
steel being employed with larger plates and fewer seams, drilled
holes, and hydraulically - closed rivets, and bad caulking
eliminated. Many small details still need insistence, however,
such as the strengthening of manholes by riveted rings, and the
use of riveted bosses for all coverings so as to avoid unseen
- leakage. The setting should be on a dry and strong site, and
should have large flues to facilitate examination.

External wasting causes many explosions, being usually con
cealed by brickwork or other covering, thus retaining moisture if
in a damp situation, and teaching the importance of narrow
surface where brickwork meets the boiler. Smooth wasting is
due to intermittent use, and is caused by moist soot, its action
being so slow and uniform as to be undetected without a drilling
I                            of the plate. Wasting also occurs at the lower part of boilers, on

account of moist ashes. The fittings cannot be too carefully
watched and tested: there should be two water-gauges, and the
safety valves be direct-loaded if possible, while the fusible-plug
metal should be renewed annually.

Corrosion or pitting is caused by the chemical action of gases
in the water, or by electric action, the plate becoming electro-
positive to the impurities, and the water, if acid or salty, forming
an electrolyte between the two. Slightly muddy water may thus
be a distinct advantage, and a little lime deposit prevent chemical
action. If the boiler needs to stand a long time it should be
.                    emptied and dried, while electric action is sometimes prevented

by suspending zinc blocks, connected by soldered wires to the
plate, which they protect by taking its place and wasting instead.

Grooving is a surface cracking due to abrupt bending under
alternate heating and cooling, and is generally found near rigid
stays. When first appearing it is called ' mechanical' grooving,
but, as it widens and deepens by corrosion, it is termed ' corro-
sional' grooving, and the obvious remedy is sufficient elasticity
with strength, while rounding stay edges at the plate.

Incrustation or scale is the hard deposit in boilers resulting
from boiling water at high temperatures. It depends on feed-
water composition, of which the following are typical analyses: